A Conductive Dinuclear Cuprous Complex Mimicking the Active Edge Site of the Copper(100)/(111) Plane for Selective Electroreduction of CO2 to C2H4 at Industrial Current Density

Inorganic solids are a kind of important catalysts, and their activities usually come from sparse active sites, which are structurally different from inactive bulk. Therefore, the rational optimization of activity depends on studying these active sites. Copper is a widely used catalyst and is expected to be a promising catalyst for the electroreduction of CO2 to C2H4. Here, we report a conductive dinuclear cuprous complex with a short Cu···Cu contact for the electroreduction of CO2 to C2H4. By using 1H-[1,10]phenanthrolin-2-one and Cu(I) ions, a dinuclear cuprous complex [Cu2(ophen)2] (Cuophen) with a remarkable conductivity (3.9 × 10−4 S m−1) and a short intramolecular Cu···Cu contact (2.62 Å) was obtained. Such a short Cu···Cu contact is close to the distance of 2.54 Å between 2 adjacent Cu atoms in the edge of the copper(100)/(111) plane. Detailed examination of Cuophen revealed a high activity for the electroreduction of CO2 to C2H4 with a Faradaic efficiency of 55(1)% and a current density of 580 mA cm−2, and no obvious degradation was observed over 50 h of continuous operation. Comparing the properties and mechanisms of Cuophen and 2 other copper complexes with different Cu···Cu distances, we found that the shorter Cu···Cu distance is conducive not only for a *CO species to bridge 2 copper ions into a more stable intermediate transition state but also for C–C coupling.


Figures S1 to S28
Tables S1 to S5 SI References Powder X-ray diffraction (PXRD) patterns were carried out on a Bruker D8 Advance diffractometer (Cu Ka).Scanning electron microscopy (SEM) images were conducted on a SU8010 system.Transmission electron microscope (TEM) images were recorded by a FEI Tecnai G2 F30 working at 300 kV.X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250 spectrometer.Elemental Analysis (EA) was carried out on an Elementar Vario EL cube. 1 H Nuclear magnetic resonance ( 1 H NMR) measurement was performed on a Bruker AVANCE-400 MHz spectrometer using DMSO as a standard.Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) spectra were recorded on a Nicolet 6700 spectrometer.Thermogravimetry (TG) analysis were recorded by a NETZSCH STA 2500 Regulus.X-ray absorption spectroscopy (XAS) measurements were colleted at the Singapore Synchrotron Light Source (SSLS) center, where a pair of chanel-cut Si (111) crystals was used in the monochromator.

Electrochemical measurements:
Working electrode preparation.The catalyst ink was prepared as follows: Cuophen (5 mg) and isopropyl alcohol (950 μL) were completely mixed.Ultrasound was performed within 10 degrees for 1 hour, and then Nafion solution (5 wt%, 50 μL) was added for another 30 minutes.Then 100 μL catalyst ink was dropped on the commercial gas diffusion layer modified carbon paper with 0.2 cm long and 1 cm wide (S = 0.2 cm 2 ), and dried completely before test.

Electrochemical measurements.
Electrochemical tests were performed on a CHI660E electrochemical workstation.In this study, all the electrochemical experiments were carried out in a three-electrode flow cell setup using 0.1 M KHCO 3 aqueous electrolyte with Pt plate as the counter electrode and Ag/AgCl electrode (PK-1038) as the reference electrode.And the working electrode was coated with evenly dispersed Cuophen catalyst on the GDL hydrophobic carbon paper.The anion exchange membrane was used to separate the cathode and anode cell.All the electrode potentials were measured against the Ag/AgCl electrode and converted to reversible hydrogen electrode (RHE) based on the following equation: During the electrochemical test, the first step was to ensure that the observable gas diffusion electrolytic cell chambers were well sealed.Then, a peristaltic pump drove the electrolyte to clean all the pipes three times.The flow of CO 2 gas was controlled by the flow meter (Alicat Mc-100sscm-d) at a rate of 20 sccm through the cavity on the back of the carbon paper, and the peristaltic pump (LongerPump, BT100-2J) flowed through the cathode and anode cavities at a constant rate of 30 rpm.Anion exchange membrane separated the two cavities to form a cathode chamber and an anode chamber.The Faraday efficiency of a certain gas product was calculated by the formula as follows: Where n = number of electrons transferred F = Faraday's constant x = mole fraction of product V = total molar flow rate of gas j Tot = total current density

Products analysis:
The gas phase collection is also collected during the electrocatalytic reduction of carbon dioxide with three electrodes.During the test, carbon dioxide flows continuously through the cathode chamber at a constant flow rate of 20 SCCM, and the outlet is connected directly to the GC system (Agilent Technologies 7890B) to collect the gas products.The products were obtained at different constant voltages and analyzed by gas chromatography equipped with two flame ionization detectors (FID) and thermal conductivity detector (TCD).The contents of gas phase products were determined by the standard curves (Figure S13).The liquid products were configured with deuterium oxide (D 2 O, 100 μl, 99.9%), dimethyl sulphoxide (DMSO, 100 μl), and electrolyte (500 μl).The liquid products were detected by 1 H NMR.

Computational methods:
Density functional theory (DFT) calculations were performed by the Materials Studio 5.5 package.
The structures of all intermediates in electrocatalysis were firstly optimized by Dmol 3 module.The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) function and TS for DFT-D correction were employed to the calculation.The convergence tolerance of energy, force and displacement convergence were set as 1 × 10 -5 Ha, 2 × 10 -3 Ha and 5 × 10 -3 Å, respectively.The core was treated using the effective core potential (ECP), and the electrons were treated by double numerical plus d-functions (DNP) basis set.

In situ attenuated total reflection infrared (ATR-FTIR) measurements:
The experimental conditions of ATR-FTIR: Firstly, the catalyst ink was prepared according to the method in the Working electrode preparation.Then, 30 uL of catalyst ink was coated on GCE and baked dry with UV lamp.The working electrode was pressed tightly on the germanium crystal for infrared signal capture, and 6.5 mL of 0.1 M KHCO 3 solution was injected into the reaction cell.After purging with high-purity carbon dioxide gas for 30 min, the data were collected at -1.4 V vs. RHE with the Ag/AgCl reference electrode and platinum wire counter electrode, and FTIR spectra were recorded manually at different times from 0 to 1500 s.Aberration-corrected HAADF-STEM showed that the distance between the two bright spots was not changed (Fig. S9), and no agglomeration was found to be copper clusters, which further indicated that Cuophen was stable.We conducted the time-dependent Faradaic Efficiencies of gaseous products and the results (Fig. S24) showed that the selectivities of gas phase products did not change significantly in the first 90 minutes, further demonstrating the stability of Cuophen in the eCO 2 RR process.Note: S 0 2 is the amplitude reduction factor; CN is the coordination number = 1.0;R is interatomic distance (the bond length between the central atoms and surrounding coordination atoms); σ 2 is Debye-Waller factor (a measure of thermal and static disorder in absorber-scattered distances); ΔE 0 is edge-energy shift (the difference between the zero kinetic energy value of the sample and that of the theoretical model).R factor is used to value the goodness of the fitting.

Fig. S3 .
Fig. S3.Photographs of bulk black crystals (a) and a single crystal (b) of Cuophen.

Fig. S8 .
Fig. S8.(a) TEM image of Cuophen catalyst.(b, c, d, e) EDS elemental mapping images showing the homogenous distribution of all four elements of Cu, C, O, and N in Cuophen.

Fig. S10 .
Fig. S10.(a) XPS spectra of Cuophen before and after the electrocatalysis.(b) Cu LMM spectra of Cuophen before and after the electrocatalysis.

Fig
Fig. S12.i-t curves of Cuophen for electrocatalytic CO 2 reduction at the potentials of -1.0 to -1.6 V vs. RHE in 0.1 M KHCO 3 .

Fig. S19 .
Fig. S19.(a) FEs of CH 4 , C 2 H 4 , CO and H 2 for Cuophen after the electrocatalysis.(b) FEs of different reduced products for Cuophen after the electrocatalysis at the potentials of -1.0 V to -1.6 V vs. RHE.

Fig. S22 .
Fig. S22.(a) Aberration-corrected HAADF-STEM image of Cuophen after the electrocatalysis.(b) Intensity profiles from the atomic sites 1 and 2 in (a).Aberration-corrected HAADF-STEM showed that the distance between the two bright spots was not changed (Fig.S9), and no agglomeration was found to be copper clusters, which further indicated that Cuophen was stable.

Fig. S23 .
Fig. S23.Anodic stripping voltammograms obtained from Cuophen-modfied glassy carbon electrode when the potential was held at -1.4 V vs. RHE in a CO 2 -saturated 0.1 M KHCO 3 aqueous solution (Scan rate: 50 mVs -1 ).To rule out the possibility of reduction of the Cu(I) ions into metallic copper and deposition on to the electrode surface, a positive potential was applied on the work electrode after the long-time electrolysis.No redox peak was observed in the LSV curve from -0.2 to 0.8 V vs. Ag/AgCl (Figure S23), indicative of the high stability of Cu(I) ions in Cuophen during the long-time electrolysis.

Fig. S24 .
Fig. S24.Faradaic Efficiencies of gaseous products in the first 90 minutes.We conducted the time-dependent Faradaic Efficiencies of gaseous products and the results (Fig.S24) showed that the selectivities of gas phase products did not change significantly in the first 90 minutes, further demonstrating the stability of Cuophen in the eCO 2 RR process.

Fig. S25 .
Fig. S25.X-ray absorption spectroscopy characterization of catalyst and references.(a) Cu K-edge in situ XANES spectra of the Cuophen measured at -1.4 V vs RHE in 1 M KOH.(b) Partially magnified in situ XANES spectra.
Crystal data and structure refinement for Cuophen.

Table S3 .
Performance comparison of various catalysts for CO 2 electro-reduction to ethylene in 0.1 M KHCO 3 .

Table S4 .
Performance comparison of various catalysts for CO 2 electro-reduction to ethylene in 1 M KOH.Energy efficiency (EE) is given for the half-cell by assuming no overpotential for the anodic oxygen evolution reaction.Therefore, EE half cell (C 2 H 4 ) = [1.23 + (-E C2H4 )] * FE C2H4 / [1.23 + (-E)], where E C2H4 is the thermodynamic potential of CO 2 RR to ethylene (+0.08 vs. RHE), E is the applied potential versus RHE and FE C2H4 denotes as the Faradaic efficiency for ethanol in percentage.

Table S5 .
EXAFS fitting parameters of the Cuophen and Cuophen-after sample measured under operando conditions.